1. Field of the Invention
This invention relates to the field of optoelectronics, and interfacing of optical fibers and thin film optical detectors; more particularly, it relates to apparatus and methods for providing registration between individual optical fibers and individual optical detectors.
2. Description of the Prior Art
Optical and electronic elements are commonly combined to form composite systems which may be referred to as optoelectronic systems; typcial examples are military target-seeking systems and systems for reading coded markings on railroad cars, wearing apparel tags and food packages. In many optoelectronic systems the optical image must be transferred from a receiving point or focal plane to another part of the system for conversion into electronic signals, usually because electronic circuitry near the receiving point or focal plane causes electrical interference with the optical-to-electronic conversion, or because there is insufficient space for the electronics associated with such conversion.
Development of optical fibers has greatly facilitated image transfer in optoelectronic systems. By means of bundles of individual optical fibers, each of which may be only a few mils in diameter and thus quite flexible, the visual image may be easily and efficiently conducted from the pick-up point or image plane to a remote location free of spurious electronic noise and where there is adequate space for associated electronics.
Normally at the image end, the fibers comprising a bundle are packed closely together; at the converting or light-emitting end, they may be spread out as necessary to accommodate to light sensitive optical detectors, generally a type of semi-conductor which may be either photovoltaic or photoconductive. If the former, the detectors, when illuminated, act as voltage sources, if the latter, the resistance of the detectors varies with their illumination. In either case, an electric analog of the illumination is provided.
Each individual fiber in a bundle generally transmits only a small portion of the total image and only a very small amount of light is transmitted and emitted by each. Consequently, there must be substantially lossless light transmission from the fiber light emitting portion to the corresponding optical detection element. Stated otherwise, very nearly perfect registration is essential between the light emitting portion of the fiber and its associated detector. Lack of such registration causes partial or complete loss of the light emitted by the fiber, with the result that the detector output signal indicates that its decoded portion of the image was darker than it actually was. While this result may be relatively inconsequential in some applications, in others, such as military target acquisition systems, it may prove critical. Also, such registration permits smaller detector areas with corresponding reduction in capacitance and improvement in response time, important factors in many practical applications.
Obtaining perfect optical registration between a very small diameter optical fiber and a generally equally small optical detector element is extremely difficult. This difficulty is greatly multiplied when, as is usual, a bundle or array of fibers is concerned. The expense of making near perfect registration may thus be prohibitive for many systems. Further, even if good registration is initially achieved, maintaining the registration under normal use conditions, which may include shocks, vibration and thermal stress, is very difficult.
Hargens III (U.S. Pat. No. 3,310,681) discloses forming of optical detectors around longitudinally etched end portions of individual optical fibers. Electrically conducting caps are applied to ends of the detectors, and insulation is applied elsewhere. Two electrical contacts are then made to each detector of each fiber. However, such a method is impractical for use on very small diameter optical fibers because of the considerable mechanical problems involved. In contrast, Dhaka (U.S. Pat. No. 3,747,127) discloses a fixed, integrated array of optical detectors separately formed on a substrate using generally known microelectronic fabrication processes, and utilizing a "flip-chip" technique. A bundle of optical fibers is terminated and clamped in spaced relationship with the associated detector array which must have the same geometrical arrangement as the bundle of optical fibers. Even assuming uniform spacing of the fibers and elements, which is unlikely, slight misalignment between the detector array and the end of the separately mounted fiber bundle, either initially or during use, will cause misalignment between every or nearly every fiber and its corresponding detecting element.
For these and other reasons, there has heretofore, to the applicant's knowledge, been available no practical, relatively simple and inexpensive method or apparatus for providing and maintaining substantially perfect registration between the light-emitting end or portion of very small diameter optical fiber, particularly arrays of such optical fibers, and associated optical detector elements.